The present invention relates to capacitive keyboards which, through the presence of a finger on a sensitive key, enable a given order or instruction to be carried out. Such keyboards are being used with increasing frequency, both for industrial and scientific applications, as well as in places open to the general public and even in electrical domestic appliances.
The known operation of such a capacitive keyboard will firstly be described in order to provide a better definition of the problem solved by the invention. Static capacitive keyboards in general terms make use of the fact that the presence of the user's finger in the vicinity of one or more conductive fittings produces electrical capacitances between the finger and the fittings and consequently modifies the existing capacitances between the said fittings.
As can be seen in a diagrammatic manner in FIG. 1, such known keyboards generally comprise sensitive keys G, each of which is associated with a pair of underlying electrodes, namely on the one hand a transmitting or emitting electrode A sequentially excited by an alternating signal supplied by a transmitting line X and on the other hand a receiving electrode B, capacitively coupled to the emitting electrode A by the corresponding sensitive key D. A receiving line Y collects on the receiving electrode B the variations in the amplitude of the alternating signal under the effect of the possible presence of the user's finger 1 in the vicinity of key G.
Fig. 2 shows the equivalent circuit diagram of such a key and it is possible to see input line X, output line Y and three capacitances C.sub.1, C.sub.1 ' and C.sub.2. Capacitance C.sub.1 represents the capacitance between electrode A and electrode G. C.sub.1 ' represents the capacitance between electrode G and electrode B, and C.sub.2 represents the direct capacitive coupling between electrode A and electrode B. The presence of the user is diagrammatically indicated by a branch 2 between the point common to capacitances C.sub.1 and C.sub.1 ' and earth. The branch 2 comprises a first capacitor 3 with a capacitance close to 4 picofarads for representing the user's finger and a capacitance 4 of approximately 60 picofarads for representing the capacitance of the user's body compared with earth. Thus, switch I diagrammatically indicates the presence or absence of the user's finger 1 on key G. The above information is based on the experimental finding that the body of a man can be represented by an electrical conductor which, compared with earth, has an average capacitance of approximately 60 picofarads when the man is wearing insulating shoes. When the finger of a user approaches the sensitive key G, it creates with the latter a capacitance which can vary between 2 and 5 picofarads, particularly as a function of whether or not gloves are worn, and the capacitive keyboard is responsible for detecting the presence of this capacitance.
FIG. 2 also shows a load impedence Z located between the receiving line Y and earth, said impedence Z diagrammatically representing the measuring electronics. In known applications of such capacitive keyboards, the two possible positions of switch I are detected by measuring the voltage collected at the terminals of Z, or the current in impedence Z, or the phase displacement between the transmitter signal at the input and the receiver signal at the output. In general terms, the presence of the user's finger corresponding to a derivation of the current by line 2, consequently leads to a reduction in the voltage on line Y due to the fact that the impedence increases between input X and output Y. It is therefore readily apparent that by examining the preceding variations on impedence Z at the output of receiving line Y, it is possible to determine in each case on which of the sensitive keys G of the capacitive keyboard has been placed the user's finger 1.
The description of the prior art will be completed by giving a few details on the way in which the different sensitive keys and transmitting and receiving electrodes of the same capacitive keyboard are connected to the outside. The easiest design arrangement for such connections is to provide a transmitting line for each trasmitting electrode and a receiving line for each receiving electrode, in which case the diagram of FIG. 2 is repeated the same number of times as there are sensitive keys G in the keyboard, However, it is clear that in the case of a relatively large keyboard this procedure leads to a very high number of connections and it is very difficult to house them in the same keyboard construction. Thus, a matrix-form supply and reading is frequently used, the different transmitting electrodes A and receiving electrodes B being distributed at the apex of a rectangular matrix in the manner indicated in FIG. 3. The various electrodes A and B are arranged in the form of a matrix network having inputs X.sub.1, X.sub.2, . . . X.sub.j for each column and outputs Y.sub.1, Y.sub.2, . . . Y.sub.i for each line. FIG. 3 is limited to the electric circuit diagram of transmitting electrodes A and receiving electrodes B. The various sensitvie keys G are not shown and are considered to be outside the plane of the drawing above each of the paris of electrodes A and B. For generating and reading the signals from the keyboard, a sequential alternating power supply is used, reaching each column X.sub.j in successive manner in the form of pulses supplying simultaneously all the electrodes A.sub.ij corresponding to the same predetermined value of j. The output signal is found by determining which of the lines Y.sub.i has a signal reduction. When this has been done, it is clear that it is key (i, j) which has been actuated if at the same time it is column X.sub.ij which has been excited by the alternating sequential pulse train. The main interest of this type of matrix keyboard is obviously the reduction in the number of wires emanating therefrom for supplying and reading the orders or instructions which it receives.
It is even possible for certain capacitive keyboards to have transmitting and receiving electrodes which are merely in the form of small metal strips constituted by the conductors of the matrix and whose crossing provides the two electrodes of each key.
It is pointed out that the interference suppression apparatus according to the invention can also be used in the case of such capacitive keyboards, whose electrodes are no longer individually specified. In the same way the capacitive keyboards of various known types, e.g. having depressable or static keys fall within the scope of the invention.
One of the most important qualities sought in capacitive keyboards is their sensitivity, which must be as high as possible in order to obtain an unambiguous response to each contact with the user's finger. However, unless special precautions are taken, there are a certain number of obstacles which prevent this from being realized.
Thus, in certain cases the users may be wearing insulating gloves, which greatly reduce the capacitance added by the finger and on the other hand unless considerable care is taken it is easy to simultaneously influence several adjacent keys, so that there are considerable doubts regarding the true intentions of the user. Moreover, it is necessary to be able to recognize a release signal with the minimum possiblity of error and even in the case of a relatively large amount of undesired background noise. However, such electromagnetic interference coming from various sources such as radio or television broadcasts, certain high voltage installations of the railways or electricity authorities and various other influencing loads can very easily swamp the useful signal in a large amount of background noise making it very problematical to interpret the release or tripping state of a capacitive keyboard.